1. Field of the Invention
The present invention relates to an imaging system utilizing a solid-state imaging device and, more particularly, to a method of constructing the transparent window of the solid-state imaging device using an optical low pass filter. The invention integrates the optical low pass filter into the solid-state imaging device to simplify the configuration of the imaging system, reducing its size and manufacturing cost. The present invention provides a solid-state imaging device having excellent frequency characteristic and a method for constructing the same by using a phase grating as the optical low pass filter.
2. Description of the Related Art
In a charge coupled device (CCD) image sensor currently widely used as an image sensor or a CMOS image sensor that has been using since 90s, the image sensors configured of light receiving elements are two-dimensionally arranged to convert input images into electrical signals.
FIG. 1 shows the configuration of a conventional camcorder or digital camera system. A motion picture or still picture 1 to be imaged is focused by an optical lens arrangement 3 and then passes through an optical low pass filter 2 to enter a light receiving element constructed on the surface of an image sensor 4. The optical lens arrangement 3 consists of appropriate optical lenses such as concave lens and convex lens in order to focus the input image 1 on the imaging device 4. The optical lens arrangement 3 or optical low pass filter of FIG. 1 usually includes an UV filter or IR filter for respectively blocking ultraviolet rays or infrared rays contained in the input image 1. The UV or IR filter is generally constructed in a manner that an appropriate material is coated on a lens or a transparent substrate. To restore the image inputted to the solid-state imaging device to the original state in the imaging system of FIG. 1, it is required that the optical low pass filter 2 has a cutoff frequency that is one-half the sampling spatial frequency.
FIG. 2 shows an ideal sampling in case where the repetitive period of the light receiving element is X in direction x and Y in the direction y in the two-dimensional image sensor. If an image having the spatial frequency spectrum of FIG. 3A is imaged using the two-dimensional sensor having the spatial sampling characteristic of FIG. 2, the sampled image has the spatial frequency spectrum of FIG. 3B in which the original image""s spatial frequency spectrum is repeated. In FIG. 3B, the frequency spectrum of the sampled image has a repetitive period corresponding to the reciprocal of the sampling interval, that is, 1/X in the x-direction and 1/Y in the y-direction. Accordingly, to restore the image inputted to the two-dimensional image sensor to the original state, it is required that an optical low pass filter which passes the spectrum corresponding to one period starting from the starting point but cuts off a spatial frequency higher than this.
As described above, to restore the image inputted to the solid-state imaging device to the original state, it is the most ideal that the optical low pass filter 2 of FIG. 1 has the cutoff frequency that is one-half the sampling spatial frequency. Here, the sampling spatial frequency corresponds to the reciprocal of the repetitive period of the light receiving element of the solid-state imager. That is, in the two-dimensional light receiving element arrangement of FIG. 2,             f      s        =                            1          d                ⁢                  xe2x80x83                ⁢        and        ⁢                  xe2x80x83                ⁢                  f          c                    =                                    f            s                    2                =                  1                      2            ⁢            d                                ,
where d is X in the x-direction and Y in the y-direction. Here, fs represents the sampling frequency and fc represents the cutoff frequency of an ideal optical low pass filter.
FIG. 4 shows the spatial frequency transfer characteristic of the optical lens arrangement. The frequency band defined by a dotted line in FIG. 4 is the frequency transfer function of an ideal optical low pass filter. The maximum transfer frequency of the lens, fm, is 2(NA/xcex). Here, NA represents the numerical aperture of the lens and xcex represents the wavelength of incident light. Though the lens functions as a kind of optical low pass filter, its maximum cutoff frequency, fm, is usually considerably higher than the ideal cutoff frequency, fc, of the low pass filter as shown in FIG. 4. The frequency transfer characteristic of the lens can approximate to the straight line of FIG. 4 to be mathematically modeled, and the difference between the approximate value indicated by the straight line and the actual transfer characteristic becomes smaller as fm becomes larger than fc.
FIG. 5A is a perspective view showing the appearance of a conventional solid-state imaging device, and FIG. 5B is a cross-sectional view showing the conventional solid-state imaging device, taken along the line Axe2x80x94A of FIG. 5A. In this conventional solid-state imaging device, the covers 51 and 52 of the solid-state imaging device chips 53 and 54 are configured of a transparent glass plate because input light should be transmitted through the covers 51 and 52, that is, transparent window, to a light receiving device placed on the surface of the solid-state imaging device chip.
FIGS. 6A, 6B and 6C illustrate conventional optical low pass filters utilizing a double refraction plate, which are currently widely used as an optical low pass filter in the conventional imaging system. Referring to FIG. 6A, an input beam incident on one surface of the double refraction plate is split into two beams, having a distance, dn, therebetween, while it passes through the double refraction plate. The relation among the thickness and refraction index of the double refraction plate and the distance, dn, satisfies the following equation:       d    n    =            t      ⁡              (                              n            e            2                    -                      n            o            2                          )                    2      ⁢              n        e            ⁢              n        o            
where t is the thickness of the double refraction plate, ne is the extra-ordinary refraction index and no is the ordinary refraction index. As shown in FIG. 6B, the conventional optical low pass filter utilizing the double refraction plate is constructed in such a manner that an x-directional double refraction plate and a y-directional double refraction plate lie in piles to enable beam splitting in the x-direction and the y-direction. An IR removal filter is generally inserted between the two double refraction plates.
In the operation of the conventional optical low pass filter utilizing the double refraction plate, the input beam, vertically incident on the surface of the filter, is split into two beams at the x-directional double refraction plate, and each of these two beams is further split into two beams at the y-direction double refraction plate. Thus, one input beam is split into four beams, arriving at the light receiving element of the solid-state imager. That is, the optical low pass filter using the double refraction plate functions as a 4-beam splitter as shown in FIG. 6C. By splitting one input beam into four beams, an image having a higher spatial frequency is converted into a lower spatial frequency before sampling of the solid state imager.
The general optical transfer characteristic function of 2-plate type double refraction plate is equals to the magnitude of the absolute value of the cosine function with the period of   1      d    n  
when it is Fourier-transformed. That is, the transfer function has a value proportional to abs(cos(2xcfx80xc3x97fxc3x97dn)) where f is spatial frequency and dn is the distance between the beams split by the double refraction plate, shown in FIG. 6A. The optical transfer function of an image which passes through the optical lens to reach the double refraction plate filter is obtained by multiplying the transfer function of the lens shown in FIG. 4 by the transfer function of the double refraction plate.
In case where the double refraction plate is applied to the conventional imaging system utilizing the solid-state imager, larger loss generates in the transfer function in a spatial frequency band lower than the cutoff frequency than in the ideal optical low pass filter. This loss deteriorates the resolution of the image sensor. Furthermore, there exists a periodic transfer function in a spatial frequency band higher than the cutoff frequency so that a higher frequency component cannot be removed. This generates aliasing to thereby bring about afterglow. In other words, the optical low pass filter utilizing the double refraction plate has larger loss in a lower band and larger surplus portion in a higher band, resulting in deterioration of resolution and poor effect on the removal of afterglow. Furthermore, the frequency characteristic of the conventional double refraction plate filter is determined by the double refraction plate""s thickness that is generally 0.5 mm. Thus, the size of the input part of the imaging system employing this becomes larger so that it is difficult to reduce the size of the system.
There have been proposed optical phase grating low pass filters having various structures for the purpose of improving the conventional optical low pass filter using the double refraction plate. U.S. Pat. No. 4,083,627 proposes the vertical grating filter, U.S. Pat. No. 4,083,627, and FIG. 7C propose the circular grating filter, U.S. Pat. No. 4, 009,939 discloses the lozenge-shaped grating filter. Further, U.S. Pat. No. 4,795,236 and No. 4,178,611 propose the parallel repetitive grating filter, and U.S. Pat. No. 4,795,236 suggests the optical phase grating low pass filter constructed in a manner that gratings having a refraction index different from that of a grating substrate whose both surfaces are used are repeatedly arranged in parallel.
However, most of the aforementioned optical phase grating low pass filters are not being actually utilized since they could not be manufactured. This is because the optical phase grating low pass filters of FIGS. 7A to 7E have the structures that two gratings having different phase shifts from each other are two-dimensionally arranged. Thus, according to computer simulation and Fourier transform carried out by this inventor, their performances are not remarkably improved compared to the conventional low pass filter using the double refraction plate. That is, the conventional optical phase grating low pass filter has the disadvantage that the transfer characteristic of the spatial frequency spectrum is not much improved compared to the conventional filter utilizing the double refraction plate because the optical phase grating low pass filter has the two gratings having different phases. Accordingly, it is difficult to reduce the size of the imaging system utilizing the conventional optical low pass filter using the double refraction plate and the solid-state imaging device. Also, its frequency transfer characteristic is not satisfactory. Moreover, the conventional optical phase grating low pass filter proposed for the purpose of improving the conventional optical low pass filter using the double refraction plate cannot be practically utilized because its frequency characteristic is poor.
It is, therefore, an object of the present invention to provide a solid-state imaging device having an optical low pass filter integrated thereinto and a method for manufacturing the same, the optical low pass filter increasing the optical transfer function at a frequency band lower than the ideal cutoff frequency corresponding to one-half the sampling spatial frequency of the solid-state imaging device and suppressing the transfer function at a band higher than the cutoff frequency.
It is another object of the present invention to provide a solid-state imaging device whose transparent window is configured of an optical phase grating low pass filter having excellent frequency transfer function and having a very thin thickness of 0.5 mm or less, thereby being realized in a small size and having satisfactory frequency characteristic.
To accomplish the objects of the present invention, there is provided a solid-state imaging device sensing input images, having an optical low pass filter integrated thereinto, the optical low pass filter suppressing a spatial frequency component higher than a specific frequency and passing a component lower than the specific frequency, the optical low pass filter being used as the transparent window of the solid-state imaging device. The optical low pass filter is an optical phase grating low pass filter.
To accomplish the objects of the present invention, there is provided a solid-state imaging device sensing input images, having an optical low pass filter integrated thereinto, the optical low pass filter suppressing a spatial frequency component higher than a specific frequency and passing a component lower than the specific frequency, the optical low pass filter being used as the transparent window of the solid-state imaging device, wherein the optical low pass filter comprises a plurality of basic arrangement patterns periodically arranged, wherein each of the basic arrangement pattern consists of: a xcfx86-phase shift grating for generating the phase shift of xcfx86, having a predetermined thickness; a 2xcfx86-phase shift grating for generating the phase shift of 2xcfx86, having a thickness twice of the xcfx86-phase shift grating; and a portion for generating the phase shift of 0, having no grating. A filter for blocking IR rays or UV rays is formed on one surface of the optical low pass filter.
To accomplish the objects of the present invention, there is provided a solid-state imaging device sensing input images, having an optical low pass filter integrated thereinto, the optical low pass filter suppressing a spatial frequency component higher than a specific frequency and passing a component lower than the specific frequency, the optical low pass filter being used as the transparent window of the solid-state imaging device, wherein the optical low pass filter comprises: a first grating for generating the phase shift of xcfx86, having a predetermined thickness and periodically arranged on one transparent grating substrate in the horizontal direction; and a second grating for generating the phase shift of xcfx86, having a predetermined thickness and periodically arranged on the other transparent grating substrate in the vertical direction, wherein the surfaces of the first and second gratings are attached to each other facing each other, to thereby construct a structure in which a xcfx86-phase shift grating for generating the xcfx86-phase shift, a 2xcfx86-phase shift grating for generating the 2xcfx86-phase shift, and a 0-phase shift grating are periodically arranged between the two transparent grating substrates. A filter for blocking IR or UV rays is formed on one of the top face and the bottom face of the attached grating substrate structure of the optical low pass filter, or a filter for blocking IR or UV rays is formed on each of the top face and the bottom face of the attached grating substrate structure of the optical low pass filter.
To accomplish the objects of the present invention, there is also provided a solid-state imaging device sensing input images, having an optical low pass filter integrated thereinto, the optical low pass filter suppressing a spatial frequency component higher than a specific frequency and passing a component lower than the specific frequency, the optical low pass filter being used as the transparent window of the solid-state imaging device, wherein the optical low pass filter comprises: a first grating for generating the phase shift of xcfx86, having a predetermined thickness and periodically arranged on one transparent grating substrate in the horizontal direction; and a second grating for generating the phase shift of xcfx86, having a predetermined thickness and periodically arranged on the other transparent grating substrate in the vertical direction, wherein the surfaces of the two grating substrates on which the gratings are not formed are attached to each other facing each other, to thereby construct a structure in which a xcfx86-phase shift grating for generating the xcfx86-phase shift, a 2xcfx86-phase shift grating for generating the 2xcfx86-phase shift and a 0-phase shift grating are periodically arranged. A filter for blocking IR or UV rays is inserted between the two grating substrates.
To accomplish the objects of the present invention, there is provided a solid-state imaging device sensing input images, having an optical low pass filter integrated thereinto, the optical low pass filter suppressing a spatial frequency component higher than a specific frequency and passing a component lower than the specific frequency, the optical low pass filter being used as the transparent window of the solid-state imaging device, wherein the optical low pass filter comprises: a first grating for generating the phase shift of xcfx86, having a predetermined thickness and periodically arranged on one surface of a transparent grating substrate in the horizontal direction; and a second grating for generating the phase shift of xcfx86, having a predetermined thickness and periodically arranged on the other surface of the transparent grating substrate in the vertical direction, wherein the first and second gratings and the grating substrate are formed of materials having the same refraction index, to thereby construct a structure in which a xcfx86-phase shift grating for generating the xcfx86-phase shift, a 2xcfx86-phase shift grating for generating the 2xcfx86-phase shift, and a 0-phase shift grating are periodically arranged.
To accomplish the objects of the present invention, there is also provided a solid-state imaging device sensing input images, having an optical phase grating low pass filter integrated thereinto, the optical phase grating low pass filter suppressing a spatial frequency component higher than a specific frequency and passing a component lower than the specific frequency, the optical phase grating low pass filter being used as the transparent window of the solid-state imaging device.